feat(random): multi-theme groundwork, locked reroll export parity, duplicate export fix, expanded diagnostics and test coverage

code/tests/test_random_determinism_delta.py

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+from __future__ import annotations
+import importlib
+import os
+from starlette.testclient import TestClient
+
+
+def _client(monkeypatch):
+    monkeypatch.setenv('RANDOM_MODES', '1')
+    monkeypatch.setenv('CSV_FILES_DIR', os.path.join('csv_files', 'testdata'))
+    app_module = importlib.import_module('code.web.app')
+    return TestClient(app_module.app)
+
+
+def test_same_seed_same_theme_same_constraints_identical(monkeypatch):
+    client = _client(monkeypatch)
+    body = {'seed': 2025, 'theme': 'Tokens'}
+    r1 = client.post('/api/random_full_build', json=body)
+    r2 = client.post('/api/random_full_build', json=body)
+    assert r1.status_code == 200 and r2.status_code == 200
+    d1, d2 = r1.json(), r2.json()
+    assert d1['commander'] == d2['commander']
+    assert d1['decklist'] == d2['decklist']
+
+
+def test_different_seed_yields_difference(monkeypatch):
+    client = _client(monkeypatch)
+    b1 = {'seed': 1111}
+    b2 = {'seed': 1112}
+    r1 = client.post('/api/random_full_build', json=b1)
+    r2 = client.post('/api/random_full_build', json=b2)
+    assert r1.status_code == 200 and r2.status_code == 200
+    d1, d2 = r1.json(), r2.json()
+    # Commander or at least one decklist difference
+    if d1['commander'] == d2['commander']:
+        assert d1['decklist'] != d2['decklist'], 'Expected decklist difference for different seeds'
+    else:
+        assert True